In accordance with social demands to energy saving in recent years, silica is compounded and used so frequently as a filler which causes low heat build-up of a rubber composition for tires to be consistent with a grip performance of a tire on a wet road surface for the purpose of saving fuel consumption of automobiles.
Silica used tends to cause aggregation of particles themselves by virtue of hydrogen bond of a silanol group which is a surface functional group, and in order to improve dispersibility of silica in rubber, a kneading time has to be extended. Also, there have been the defects that a silica-compounded rubber composition is elevated in a Mooney viscosity due to insufficient dispersibility of silica in rubber and is inferior in processability such as extrusion. Further, the surface of silica particles is acidic, and therefore silica involves the defects that it adsorbs basic substances which are used as a vulcanization accelerator to prevent the rubber composition from being sufficiently vulcanized and that the storage modulus of the rubber composition is not enhanced. Accordingly, silica-compounded rubber compositions have so far been desired to be improved in processability and the like.
1) A technology in which amide compounds (fatty acid amides) having a polar end showing a weak chemical reactivity to silica and a non-polar end showing a weak chemical reactivity to elastic polymers are added to silica-compounded rubber as a processing aid for enhancing dispersibility of a reinforcing silica filler in a rubber composition (refer to, for example, patent document 1), and 2) a technology in which tertiary amine compounds are added to silica-compounded rubber to thereby improve dispersibility of silica (refer to, for example, patent document 2) have so far been known as technologies for improving processability and the like in a silica-compounded rubber composition.
It is described in the foregoing patent document 1 that amide compounds (fatty acid amides) having structures which are different from those of the compounds used in the present invention are added to silica-compounded rubber to thereby improve processability thereof, but the problem that the vulcanization rate is delayed is involved therein. Also, it is described in the foregoing patent document 2 that tertiary amine compounds having structures which are different from those of the compounds used in the present invention is added to silica-blended rubber to thereby improve processability thereof, but in this case, the problem that the scorch time is expedited to bring about rubber burning is involved therein.
On the other hand, known are sulfur blooming inhibitors for rubber comprising specific alkanolamide compounds and a technology for compounding the sulfur blooming inhibitors with rubber compositions (refer to, for example, patent document 3). However, improvement in the processability in compounding silica is not described therein, and the invention is different in the subjects of the invention, use applications and a technical concept from the present invention.